1. Field of the Invention
The present invention relates to a drive method and a drive device for a display panel unit that displays images.
2. Description of the Related Art
At present AC-type (alternating current discharge type) plasma display panels are increasingly used in commercial products as thin or flat display devices. The discharge cells in a plasma display panel correspond to pixels of images to be displayed, and emit light using the discharge phenomenon. This means that each discharge cell has only two levels: a light-emitting state corresponding to the highest brightness level and a lights out state corresponding to the lowest brightness level. Grayscale driving using subfields is therefore used in plasma display panels of this type in order to achieve intermediate levels of display brightness faithful to the input image signal.
In grayscale driving based on the subfield method, each field for image signal is divided into a plurality of subfields, each subfield is assigned its own light-emission repetition number (how many times the subfield concerned emits light or how long the subfield emits light), and display driving is carried out for such one field worth of image signal. An address process and a sustain process are carried out successively within each subfield. During the address process, selective discharge takes place within each discharge cell, in response to the input image signal, so as to form a wall charge of a specified amount (or to eliminate the wall charge). During the sustain process, only those cells in which the specified amount of wall charge has been formed are repeatedly discharged, so as to maintain light emission that goes with that discharge. At least within the leading subfield, a preliminary formatting process (initialization process) precedes the address process. The preliminary formatting process resets all the discharge cells at once by causing discharge in all the discharge cells (called “reset discharge”), thereby forming a specified amount of wall charge within all the discharge cells (or eliminating a wall charge from all the discharge cells).
Because this reset discharge is produced in all discharge cells independently of the content of the image to be displayed, the light emission caused by this discharge tends to lower the contrast of the image.
Japanese Patent Kokai (Laid-open Application) No. 2001-312244 discloses a drive method in which no reset discharge for preliminary formatting purposes is carried out on discharge cells which emit light at 0-level brightness. In this drive method, selective discharge is produced for each discharge cell that is to emit light, in the address process of each subfield. By producing a specified amount of wall charge within the discharge cells in this selective discharge, these discharge cells become ready for discharge in the subsequent sustain process.
However, when 0-level brightness continues within a discharge cell for a long period of time, the wall charge remaining inside that discharge cell gradually decreases under the influence of dark current and other factors, so that even if selective discharge takes place in this discharge cell during a later address process, the amount of wall charge formed will not reach the specified (desired) value. As a result, there is a risk with this method that some discharge cells might not discharge as they ought to during the sustain process, and that an incorrect or inadequate image might be created.